Conventionally, there is proposed an optical scanning device that has a BD (Beam Detector) sensor for detecting laser light scanned by a polygon mirror to output a BD signal. This optical scanning device controls a writing timing of laser light by using the BD signal. For example, in a technology disclosed in JP-A-4-240960, there is provided a unit that generates and outputs a signal indicating one of an immediate pre-image formation region (state A), an image formation region (state B), an immediate post-image formation region (state C), and a BD signal detection region (state D). In this technology, only the BD signal detected at the time of the state D is used as a normal BD signal.